Engine oil has not only the function of lubricating the engine, but also various functions, including those of cleaning, rust prevention, and corrosion control. After engine oil is degraded, these functions cannot be maintained, and formation of sludge and the like can cause the trouble of the engine, such as damage thereto. Therefore, it is preferable to replace the degraded oil early depending on the degree of degradation thereof. On the other hand, from the viewpoint of environmental protection, it is demanded to reduce the amount of waste oil, and particularly in the case of engine oil, it is desired to prolong the intervals of replacement of engine oil due to the large volume of waste oil and the high frequency of the replacement. From the above-mentioned viewpoint of engine protection and environmental protection, it is a very important theme to accurately determine actual degradation of engine oil and appropriately set the time for replacement of engine oil.
Therefore, conventionally, there have been proposed various degradation determining devices concerning engine oil, and for example, one disclosed in Patent Literature 1 is known. This degradation determining device includes a first determination device that carries out determination according to properties of engine oil (hereinafter simply referred to as “oil”), and a second determination device that carries out determination according to information on engine operation. When either of the first and second determination devices determines that the oil is degraded, a display displays a notice that the oil is degraded to urge the driver to replace the oil.
The first determination device uses an optical sensor which emits light from a light emitting part thereof toward oil, and receives light reflected from the oil at a light receiving part thereof. When the amount of received light is smaller than a first predetermined reference value, it is judged that particles having relatively large sizes are generated within the oil, and hence it is determined that the oil is degraded. On the other hand, the second determination device calculates a cumulative value of the information on engine operation, such as mileage of an automotive vehicle, after oil replacement, and when the calculated cumulative value becomes equal to or larger than a predetermined second reference value, it is determined that the oil is degraded. Further, the above-mentioned first reference value for the first determination device is set to be more strict from the view point of using oil in a good condition, whereas the second reference value for the second determination device is set to be less strict from the viewpoint of using the oil to a limit within which the oil does not cause any engine trouble.
However, the conventional degradation determining device adopts the result of determination by the first determination device that uses the first reference value which is more strict, provided that the first determination device is normal. Therefore, it is likely to be determined that the oil is degraded even when the degree of oil degradation is not so high, which causes the oil to be replaced too early, causing wasteful disposal of the used oil.
Further, when the first determination device is faulty, the result of determination by the second determination device which uses the second reference value is adopted as a backup. The method of determination employed by the second determination device, however, only estimates the degree of oil degradation according to the cumulative value e.g. of mileage after the replacement of oil. In contrast, the actual progress of degradation of oil largely differs depending not only on the mileage or cumulative value of the number of rotations of the engine, but also on the environment and conditions of operation of the engine. This makes it impossible for the second determination device to accurately determine the degree of oil degradation. Therefore, to more positively avoid the troubles of faulty lubrication and the like, it is necessary to set an extra safety factor to the second reference value, which makes the time of replacement of oil earlier.
Further, according to the conventional degradation determination device, the optical sensor is required to be provided for degradation determination by the first determination device, which accordingly increases the manufacturing costs.
Further, as another conventional degradation determining device concerning engine oil, one disclosed in Patent Literature 2 is known. This degradation determining device pays attention to an amount of antioxidant remaining in engine oil (hereinafter referred to as “oil”) as an indicator for use in determining degradation of the oil, and the remaining amount of antioxidant is detected using an infrared spectrometer. In the degradation determination device, the infrared spectrometer is disposed in a bypass passage connected to a downstream side of an oil filter in an oil passage, and the infrared spectrometer determines an infrared absorbance of a wavelength indicative of a peak characterizing an infrared absorbance spectrum of the antioxidant. The remaining amount of antioxidant is calculated based on the absorbance. Thus, the degradation of oil is determined based on the thus calculated remaining amount of antioxidant.
However, in the conventional degradation determination device, it is required to use the infrared spectrometer, which is expensive, to determine the remaining amount of antioxidant, resulting in an increase in the manufacturing cost of the device.
The present invention has been made to solve the above problems, and a first object thereof is to provide an engine oil degradation-estimating device which is capable of determining degradation of engine oil inexpensively and accurately, thereby making it possible to properly determine the time for replacement of the engine oil.
Further, a second object of the invention is to provide a device for estimating an antioxidant performance of engine oil, which is capable of accurately determining the antioxidant performance of engine oil, and thereby properly determining degradation of engine oil and time for replacement thereof, without using an expensive sensor.    [Patent Literature 1] Japanese Laid-Open Patent Publication (Kokai) No. H07-189641.    [Patent Literature 2] Japanese Laid-Open Patent Publication (Kokai) No. H08-226896